1. Field of the Invention
The present invention relates to a production method for producing a hydrogen adsorption alloy and an apparatus for use therein.
2. Prior Art
Various techniques have been heretofore developed in which hydrogen is adsorbed in a certain metal or alloy to be stored therein and transferred therefrom in the form of metal hydride, and those techniques have been further applied to such practical use as the purification of hydrogen, heat pumps, air conditioning systems, etc. In such practical applications, because an exothermic reaction or an endothermic reaction takes place without fail at the time when the metal hydride adsorbs or discharges hydrogen, such reactions or property can be utilized in heat exchanger, heat pump, etc.
Several combinations of metals forming hydrogen adsorption alloys have been proposed up to today and some of them have been actually put into practical use. Those combinations of metals are Mg--Ni, Mg--Cu, Ca--Ni, Fe--Ti, Ti--Mn, La--Ni, Mischmetal--Ni, etc. A large number of other alloys composed by substituting a different metal for a part of the mentioned metals have been also disclosed including Mg.sub.2 Ni.sub.0.75 Cr.sub.0.25, TiFe.sub.0.8 Mn.sub.0.2, Ca.sub.0.7 Mn.sub.0.3 Ni.sub.5, LaNi.sub.4.7 Al.sub.0.3.
Generally speaking, a hydrogen adsorption alloy is produced by alloying one or more metals selected from a group including Mg, Ca, La, Mischmetal, Ti, etc. with one or more metals selected from a group formed by Ni, Al, V, Cr, Fe, Co, Zn, Cu, Mn, etc.
To produce a hydrogen adsorption alloy, different kinds of metals used as material are dissolved in a high frequency induction furnace or an arc type high temperature melting furnace. A high frequency induction furnace is suitable for mass production, but it is essentially required for this furnace to be adjusted to an inert atmosphere using Ar gas or the like in view of the prevention of metals from oxidation because some of the metals, in particular, Mg, Ca, Al, etc. have a high vapor pressure when vaporized and have a strong affinity to oxygen.
After an alloy of desired composition has been obtained through the steps of dissolving the material metals to be mixed with each other and proceeding with every necessary reaction under a high temperature, then a casting is formed in a metal mold under a non-oxidizing atmosphere to obtain an ingot. The obtained ingot is then subject to a heat treatment, and after completely forming a solid alloy, the alloy is crushed in a crusher under a non-oxidizing atmosphere, whereby fine particles of hydrogen adsorption alloy of desired particle size are obtained.
It has also recently been proposed to obtain an alloy of desired composition keeping a solid state without dissolution. This method is generally referred to as the "Mechanical Alloying Process" which was originally developed by Benjamin of INCO U.S.A. in the 1970s. In this process, mechanical energy is applied to metal fine particles by means of a high energy ball mill (Attritor) and ultra-fine particles are dispersed by repeating cold press-fitting and breakdown.
With respect to the principle of this mechanical alloying, it is generally understood that the process comprises a series of steps of: forging fine particles by large impact milling with the particles being flattened and flaked; breaking down or peeling the hardened particles and repeating cold forge welding (kneading); growing a lamella structure between the alloy components thereby forming rapidly fine crystal grains so that particles of one metal may be dispersed into those of another metal; and equiaxing shaping of the particles thereby achieving a randomization.
M. Y. Song and E. I. Ivanov published the results of an experiment on alloying fine particles of Mg and Ni using the Mechanical Alloying Process by means of a planetary ball mill (Hydrogen Energy Vol 10 No. 3 P. 169-178, 1985).
It was reported that, in this experiment, acceleration of the planetary ball mill was established to be 0.6 G, and a carbonyl type Ni was employed and mixed with Mg under an Ar gas atmosphere for 30 minutes to obtain samples. The samples were then subjected to various hydrogen treatments and comparatively discussed after X-ray diffraction. According to the report, it was acknowledged that among the samples on which hydrogen treatment was repeated from 1 to 58 times, phases of Mg.sub.2 Ni and a very small amount of MgO, Mg, Ni were detected existing in a mixed state in the samples subjected to less hydrogen treatment, and therefore heat treatment (annealing) was applied thereto. on the other hand, in the samples subjected to a larger number of hydrogen treatments, most of the Mg and Ni is transformed to Mg.sub.2 Ni. As a result of this, it was reported that heat treatment was more effective than repetition of hydrogen treatment. Thus, a production method of hydrogen adsorption alloy without depending upon dissolution was disclosed for the first time, though it was not a perfect method.
It is well known that production of hydrogen adsorption alloy by the conventional method requiring the step of dissolution requires high level techniques as well as sufficiently controllable equipment. For example, in the case of producing Mg.sub.2 Ni, the vapor pressure of Ni fluctuates at a high level such as 2057 mmHg at 10.degree. C., 2732 mmHg at 760.degree. C., and the vapor pressure of Mg fluctuates from 743 mmHg to 1107 mmHg. The vapor pressure of Ca fluctuates similarly from 983 mmHg to 1487 mmHg. Under such a wide range of fluctuation, it is very difficult to increase furnace temperature while keeping the vapor pressure well-balanced. From the viewpoint of dissolution, the question of whether the degree of solid solution of both components is high or low may more or less influence alloying and make it difficult for alloying, but what is most important is the difference between two components relative to the density aspect and the melting point. They are respectively 8.90 g/cm.sup.3 and 1455.degree. C. in Ni, 1.74 g/cm.sup.3 and 650.degree. C. in Mg, and 1.55 g/cm.sup.3 and 850.degree. C. in Ca, which clearly shows the difficulty in alloying Mg or Ca with Ni. On the other hand, the density and melting point of La are respectively 6.15 g/cm.sup.3 and 826.degree. C. This density of La may be approximate to that of Ni and reduce the difficulty to a certain extent. However, rare earth metals are very precious and expensive resources.
A serious problem in alloying Mg and Ni exists in that the vapor pressure of Mg reaches approximately 25 Kg/cm.sup.3 which is near the melting point of Ni, and because of such a high pressure it is difficult to prevent molten metal from vaporizing and bringing about excess Ni, eventually resulting in a product wherein MgNi.sub.2 is not partially hydrated. If mixing excessive Mg to prevent such a problem, there arises a different problem of containing separate the single substance of Mg, the chemical structure of which is nominally Mg.sub.2.35 Ni but actually Mg.sub.2 Ni+Mg.sub.0.35.
The mentioned problems affect the characteristics of the obtained hydrogen adsorption alloy as described hereunder with reference to FIGS. 9 and 10.
FIG. 9 is a pressure-composition isothermal line diagram (hereinafter referred to as "PCT line diagram") of a hydrogen adsorption alloy Mg.sub.2.35 Ni obtained by the method including the dissolving step (hereinafter referred to as "dissolving method"). In this drawing, the ordinate axis indicates the hydrogen pressure P (in MPa) and the abscissa axis indicates the atomic ratio N/M of hydrogen gas to metal so as to show the behavior of the atomic ratio according to the adsorption and discharge of hydrogen gas at a fixed temperature (350.degree. C.).
The curve in this drawing indicates that when hydrogen has reached nearly 0.5, both adsorption and discharge are clearly divided into a portion A of gentle inclination rightward and a portion B of almost horizontal direction also rightward. The portion A shows adsorption and discharge of hydrogen in and from a single substance, hydrogen, and the portion B shows adsorption and discharge of hydrogen in and from Mg.sub.2 Ni. In other words, the existence of the portion A means that there is Mg to be combined with hydrogen gas, and that Mg for inferior to Mg.sub.2 Ni in view of its affinity is contained in the alloy, resulting in a decline of its function as a hydrogen adsorption alloy.
FIG. 10 is a high pressure thermal difference analysis diagram (hereinafter referred to as "DTA line diagram") of the same sample as FIG. 9, and in which the ordinate axis indicates temperature and the abscissa axis indicates time. Curve C shows temperatures obtained by measuring Mg.sub.2.35 Ni charged in a container after sealing hydrogen of a certain pressure (1.1 MPa) in the container, heating the container from outside to 500.degree. C. at its highest otherwise cooling it down from 500.degree. C. Curve D shows the temperature difference generated between the sample and a reference sample (alumina) charged in the container for comparison. Since an exothermic reaction takes place at the time of adsorbing hydrogen gas and an endothermic reaction takes place at the time of discharging it, a downward peak corresponding to the discharge is seen at the time of heating, while an upward peak corresponding to the adsorption is seen at the time of cooling, in the curve D. However, as is clearly shown by points P, Q and R, when looking carefully, it is found that each peak is in the form of a double peak or refracting points similar to double peaks and not a single peak form. And this means that phase changes take place not only between Mg.sub.2 Ni and Mg.sub.2 NiH.sub.4 but also between Mg and MgH.sub.2. Such phase change between Mg and MgH.sub.2 occurs because Mg dissociates at higher temperature than Mg.sub.2 Ni under the same hydrogen pressure.
FIGS. 11(a) and (b) show other data supporting the above fact, and in which FIG. 11(a) is a mapping diagram of Ni and FIG. 11(b) is the same diagram for Mg both prepared by an electronic probe microanalyzer.
In FIG. 11(a) the white dots represent the existence of Ni, and the more the density of white dots are, the more Ni exists, while the black portions show the nonexistence of Ni. In FIG. 11(b) the white portions show the existence of Mg while the black portions show the nonexistence of Mg.
In FIGS. 11(a) and (b), the distribution of Ni and Mg is not even but partial, which means that alloying of Mg.sub.2 Ni is insufficient resulting in the existence of single phase Mg and Ni.
In effect, a serious problem of hydrogen adsorption alloy produced by the mentioned dissolving method exists in that components negatively affecting the quality of the alloy still remain in mixture in addition to the production difficulty thereof.
On the other hand, the attempt to obtain Mg.sub.2 Ni by the so-called mechanical alloying method instead of the dissolving method mentioned above suggests a certain technical feasibility. It was recognized, however, that after repeating hydrogeneration and dehydrogeneration of sample alloy under the conditions of 0.7 MPa and 300.degree. C., single phase Mg or Ni did not disappear by simply repeating hydrogeneration several times, but that the sample alloy could be almost entirely transformed to Mg.sub.2 Ni only after completing a heat treatment in which the temperature was kept at 270 to 300.degree. C. for a long time of two months and repeating hydrogeneration 58 times. Though this method is called the mechanical alloying method, the state of the art has not reached yet a satisfiable level of alloying of different kinds of metal. After all it may be a reasonable evaluation that this method still remains at a level wherein oxides of the same metal system are simply dispersed in the form of ultra-fine particles in metal particles, or the phase of the starting material of metal composition is changed to a different one (amorphous phase, for example).